1. Field of the Invention
The present invention relates to a method and apparatus for generating a Channel Quality Indicator (CQI) in a wireless communication system. More particularly, the present invention relates to a method and apparatus for generating a CQI in a wireless communication system to which Adaptive Modulation and Coding (AMC) is applied.
2. Description of the Related Art
Generally, wireless communication systems use an AMC scheme which improves spectral efficiency by adaptively changing a modulation order and an error correction code according to the quality of a channel between a transmitting end and a receiving end. That is, the AMC scheme is used to achieve maximum throughput with a given frequency band. To use the AMC scheme, a CQI estimated at the receiving end needs to be fed back to the transmitting end.
In recent communication systems, Hybrid Automatic Repeat Request (HARQ) as well as AMC is generally used and both of them are used in the 3rd Generation Partnership Project Long Term Evolution (3GPP LTE) standard. The HARQ scheme retransmits only a transport block in which an error is found during initial transmission, thereby reducing a final error rate of the transport block and thus enhancing spectral efficiency.
The following description regarding the related art will be made based on the 3GPP LTE standard, and assumes the use of a spatial multiplexing Multiple Input Multiple Output (MIMO) communication system to which the AMC scheme is applied.
FIG. 1 is a block diagram showing a structure of a transmitter of an LTE-based MIMO system to which AMC is applied according to the related art.
First, a transmitting flow of control information 100 will be described.
The control information 100 includes Modulation and Coding Scheme (MCS) information and traffic channel resource allocation information which are determined for the AMC for each transport block for transmission of a traffic channel, the number of transport layers, precoding matrix index information, and so forth. To the control information 100 is added a Cyclic Redundancy Check (CRC) 102 which is an error detection scheme for detecting an error occurring during transmission. The control information 100 is encoded at a Forward Error Correction (FEC) encoder 104 using an FEC code which is an error correction code for correcting an error caused by noise. The encoded information bits are mapped to signal constellations at a Modulation (MOD) unit 106 and then delivered to a resource allocator 108. An interleaver may exist between the FEC encoder 104 and the MOD unit 106, though not shown in FIG. 1.
A description will now be made of a transmitting flow of traffic information 110.
The traffic information 110 is divided into transport blocks at a Serial-to-Parallel (S/P) converter 112. A transport block is a unit to which MCS is applied. To the divided traffic information is added a CRC 114 which is an error detection scheme for detecting an error occurring during transmission. The CRC-added traffic information is channel-encoded at an FEC encoder 116 using an FEC code which is an error correction code for correcting an error caused by noise. The channel encoding is performed based on the determined MCS information, and thus channel-encoded bits are output. Since the number of channel-encoded bits is generally not matched to the number of modulation symbols allocated to each user, it is matched to a proper one at a rate matcher 118 using the determined MCS information and the traffic channel resource allocation information. A modulation (MOD) unit 120 maps the rate-matched bits to signal constellations. A precoder 122 precodes output symbols of the MOD unit 120 using feedback information such as a CQI. An Inverse Fast Fourier Transform (IFFT) unit 124 transforms the precoded information from a frequency domain to a time domain, and delivers it to a Radio Frequency (RF) converter (not shown).
FIG. 2 is a block diagram showing a structure of a receiver corresponding to a transmitter shown in FIG. 1 according to the related art.
A signal received by each receiving antenna is transformed from a signal in the time domain to a signal in the frequency domain by a Fast Fourier Transform (FFT) unit 200, and the transformed received signal is divided into a control channel signal and a traffic channel signal at a resource deallocator 202.
A control channel detector 204 detects MCS information (transmitted from a transmitting end) necessary for traffic channel reception, traffic channel resource allocation information, the number of transport layers, precoding matrix index information, and so forth from the control channel signal, using the control channel signal received from the resource deallocator 202 and a predetermined channel estimation value.
An effective channel generator 206 calculates an effective channel value, reflecting an influence of precoding, using the detected number of transport layers and the detected precoding matrix index. If a Reference Signal (RS) used for channel estimation is commonly used for terminals, the effective channel generator 206 would be required because the influence of precoding is not reflected in the channel estimation value. However, if a dedicated RS for channel estimation is allowed (that is, an RS allocated to each user is transmitted after being precoded), the effective channel generator 206 is not required because the influence of precoding has already been reflected in the channel estimation value.
A MIMO demodulation (DEMOD) unit 208 generates a Log Likelihood Ratio (LLR) value using the detected number of transport layers, the effective channel value, and the traffic channel signal transmitted from the resource deallocator 202. The generated LLR value is rate-dematched at a rate dematching unit 210 and then delivered to an FEC decoder 212 for decoding, thus outputting decoded information bits. The decoded information bits then undergo error detection and retransmission request identification at a CRC check unit 214. If there is no error in the decoded information bits, they are transmitted to an upper layer after being parallel-to-serial converted at a Parallel-to-Serial (P/S) converter 216.
A CQI metric generator 218 estimates a CQI metric value using the effective channel value generated in the effective channel generator 206, and transmits the estimated CQI metric value to a CQI generator 220. The CQI metric value refers to a value representing the performance of a traffic channel and a real-time channel status, such as an effective Signal-to-Interference Noise Ratio (SINR) or capacity. In an alternative exemplary embodiment of the present invention, the CQI metric generator 218 may generate the CQI metric value using output LLR values 230 and 232 of the MIMO DEMOD unit 208, instead of the effective channel value output from the effective channel generator 206.
The CQI generator 220 generates a final CQI index to be reported by quantizing the CQI metric value transmitted from the CQI metric generator 218.
A CQI offset generator 222 determines a CQI offset using Acknowledgement (ACK)/Negative Acknowledgement (NACK) information indicating an error detection result output from the CRC check unit 214, and transmits the determined CQI offset to the CQI generator 220. The CQI offset is used for the CQI generator 220 to generate the final CQI index.
When a CQI metric value transmitted from a CQI metric generator 218 is μ, a CQI offset value transmitted from a CQI offset generator 222 is δ, and spectrum efficiency or frequency error rate is not considered, then a CQI index σidx generated by the CQI generator 220 may be expressed by:σidx=max{σ|μ−δ>fth(σ)}  (1)orσidx=(max{σ|μ>fth(σ)})−δ  (2)
where fth(σ) indicates a threshold value for a CQI index of σ. According to antenna correlation or a Multiple Input Multiple Output (MIMO) scheme used for transmission, several threshold tables may exist for fth(σ). An example of the MIMO scheme may be a transmission mode according to the Long Term Evolution (LTE) standard. Since a minimum operating time is required to adjust the Modulation and Coding Scheme (MCS) using a reported CQI index, the CQI metric generator 218 includes a prediction filter considering the operating time.
The foregoing conventional scheme generates the CQI offset and adaptively operates according to the CQI offset, but may have a limitation in optimizing spectral efficiency due to a difficulty in prediction of a Frame Error Rate (FER) of the CQI index to be reported, a channel environment estimation error, a limited threshold table, and so forth.